Aromatic polyimides have found extensive use in industry as fibers, composites, molded parts and dielectrics due to their ease of coating, toughness, flexibility, mechanical strength and high thermal stability. In the electronic industry, polyimides have proven to be useful due to their low dielectric constant and high electrical resistivity. Such polymers have been used in both film and coating form as advanced materials for such uses as interlevel dielectrics, passivation coatings, insulating coatings, die attach adhesives, flexible circuit substrates, and the like.
Many electronic applications, for example, passivation coatings or interlevel dielectrics, require that vias (or openings) be etched through the polymer coatings to permit access for electrical connections that run between the substrate and the outside environment.
Etching vias through polyimides, or their polyamic acid precursor, requires a multistep procedure. The polymer is generally dissolved in solution and the resulting solution of polymer is spread on a substrate to form a coating. In the case of polyamic acid, the coating is further coated with a photoresist material which itself is in a solvent and that solvent is removed, typically by heating (also called soft baking). The photoresist material is then shielded with a mask containing a pattern of openings and the photoresist material is exposed to actinic radiation. Thus, the photoresist material is photochemically altered such that the areas that were exposed to actinic radiation are soluble and vias (or openings) are created by taking advantage of this selective solubility to develop and remove specific areas of photoresist material. Then, the polyamic acid coating can be etched. After the polyamic acid is etched, forming vias in the polyamic acid coating, the remaining photoresist material is removed. Thereafter, the polyamic acid is iodized, generally by heating, generally in a range of from about 200.degree. C. to about 400.degree. C., to form the final coating.
Thus, etching can be viewed as a multistep process to dissolve selected areas of polyamic acid coating on a substrate with an appropriate solvent to form vias (or openings) in the coating. However, the number of steps involved in the process would be substantially reduced if photosensitivity could be incorporated into the polymer so that a photosensitive polymer could be applied to the substrate and patterned directly, i.e., without the need for the application and removal of a photoresist material. The number of steps would be further reduced if the polymer coating could be applied as a polyimide, thereby eliminating the imidization step. By reducing the number of process steps, the production of each electronic component will take less time and overall production will be more efficient.
Early attempts to provide photosensitive polymers used photosensitive polyamic acid derivatives. For example, several known polyamic acid derivatives are based on the formation of a polyamic ester. Extensions of this technology are found in U.S. Pat. Nos. 4,416,973 (photocrosslinking olefinic groups), 4,430,418 (halogen-containing amines), and 4,454,220 (polyamic acid). In general, the photosensitive polyamic acid precursor is formed by first reacting a dianhydride, such as pyromellitic dianhydride (PMDA), with an allylic alcohol to form a diacid-diester. The diacid-diester is reacted with thionyl chloride to form the bis-acid chloride. Reaction of the acid chloride with a diamine, such as oxybisaniline (OBA), yields the photosensitive polyamic ester. Upon irradiation, the olefinic groups undergo a photochemically allowed (2+2) cycloaddition reaction. This cross-links the polyamic ester and reduces the solubility of the area that was exposed to radiation. After development, a high temperature cure (400.degree. C.) results in elimination of the cross-linking groups and imidization of the polyamic acid to form the polyimide, in this case pyromellitic acid dianhydride/oxybisaniline (PMDA/OBA). It is theorized that the majority of the cross-linking groups are lost as the allylic alcohol (Ahne et al., Proc. of the ACS Div of Polymeric Materials: Science and Engineering, Vol.55, 406-412 (1986)). It is this loss of allylic alcohol that is believed to be responsible for much of the shrinkage in these materials, ranging from 40 to 60 weight percent.
Shrinkage is a significant problem which may cause delamination of the film from the substrate because of the internal stress that builds up in the film from the polymer shrinkage. Further, in order to compensate for polymer shrinkage, the mask design must be adjusted so that the final pattern features have the correct dimensions. For example, in order to pattern 10 micron features from a polymer having a 45 percent thickness loss and a 20 percent loss in linewidth, one would need a 12 micron pattern. Further, the degree of feature shrinkage can vary from one polymer to another; therefore, the shrinkage characteristics of each new polymer must be determined in order to compensate for that polymer's unique characteristics.
Some have tried to overcome the shrinkage problem by providing a cured polyimide with photoimageable properties incorporated into the polymer backbone. Rohde, O., 3rd Annual International Conference on Crosslinked Polymers, Luzern, Switzerland, 197-208 (1989) , discloses a photoimageable polyimide prepared from pyromellitic acid dianhydride (PMDA) and 2,2',6,6'-tetramethyl-4,4'-methylenedianiline (TMMA).
U.S. Pat. No. 4,912,197 discloses transparent-to-clear aromatic polyimides. The polyimide 2,2-bis(3,4dicarboxyphenyl)-hexafluoropropane dianhydride/2,3,5,6-tetramethyl-1,4-phenylene diamine (6FDA/DMDE) is disclosed; However, this reference does not teach the present invention. In fact, this reference specifically requires the presence of 3,3',4,4'-benzophenone tetracarboxylic acid dianhydride (BTDA) to provide photochemical crosslinking. Although the present invention provides that a photosensitizing moiety, such as BTDA, may be incorporated into the polymer to increase photosensitivity, photoimageability does not depend on the presence of the photosensitizing moiety.
U.S. Pat. Nos. 4,705,540 and 4,717,394 disclose 6FDA/DMDE-containing polyimides for gas separation membranes. U.S. Pat. No. 4,717,393 discloses auto-photochemically crosslinkable gas separation membranes. This reference discloses from 5 to 100 percent BTDA and attributes the photo-crosslinking to absorption of photochemical energy by the benzophenone chromophore.
U.S. Pat. No. 4,629,777 discloses photoimageable polyimide systems from BTDA and aromatic diamines which carry ortho-aliphatic substituents. However, this reference does not teach the use of 6FDA, which is considered to be important to provide the transparency and low color found in the photoimageable polyimide coatings of the present invention.
U.S. Pat. No. 4,656,116 discloses radiation-sensitive coating compositions that crosslink on exposure to actinic radiation. However, this reference does not teach the use of 6FDA.
Other references disclose 6FDA-containing polyimides, for example, U.S. Pat No. 3,822,202 discloses gas separation membranes containing 6FDA, BTDA, and generally substituted phenylene diamines. U.S. Pat. No. 3,356,648 generally discloses polyamic acids and polyimides from hexafluoropropylidine-bridged diamines. U.S. Pat. No. 3,959,350 discloses a melt-fusible linear polyimide of 6FDA and m- or p-phenylene diamines. Published Japanese Application 63/333,746 discloses a resin film pattern formation method for polyisoimides which are derived from various diamines and dianhydrides, including 6FDA.
While these references generally disclose photoimageable polyimides, they do not disclose the photoimageable polyimide coatings of the present invention having the properties described.